Polycera quadrilineata has no common name and is very abundant most years around Britain wherever there is kelp. They appear sometime in Spring and feed on sea-mat, Membranipora membranacea, a colonial animal that grows on kelp fronds. By the end of July most have laid their eggs and died. Here is a mating group with an egg mass in the foreground and patches of sea mat to the right.
North Sea. St Abbs Marine Reserve